This project will investigate whether neurons in the suprachiasmatic nucleus of the hypothalamus (SCN) that colocalize vasoactive intestinal peptide (VIP), peptide histidine isoleucine (PHI) and gastrin releasing peptide (GRP) mediate the synchronization of circadian rhythms with the day-night cycle. We propose a 'ratio hypothesis' which states that light communicated to the SCN via afferent pathways sets the ratio of VIP/PHI (VIP and PHI are derived from the same precursor and exhibit a 1:1 ratio in the SCN) to GRP available for release from SCN neurons by altering the cellular levels of VIP/PHI mRNA and/or GRP mRNA, and that the ratio of VIP/PHI to GRP released from SCN neurons in response to light determines how the circadian clock is reset by light. The following specific aims will critically test this hypothesis. Specific Aim 1 will use quantitative in situ hybridization to investigate whether light regulates VIP/PHI and GRP gene expression within the SCN. Specific Aim 2 will determine whether the ratio of VIP/PHI to GRP coreleased within the SCN determines how the circadian clock is reset by examining whether antagonists to VIP and GRP block the ability of light to reset the clock, and by determining the resetting effects of the microinjection of different ratios of VIP/PHI to GRP into the SCN. Specific Aim 3 will define the SCN neurons that are targets of VIP/PHI/GRP by identifying the neurotransmitters produced by SCN neurons that contain GRP receptor mRNA. In addition to providing a better understanding of the neurochemical mechanisms underlying circadian control, this project will also provide fundamental new information about the regulation of colocalized neuropeptides and the functional significance of neurotransmitter corelease. The information provided by this project should contribute to the development of new treatments for disorders of the nervous system, particularly those linked to the circadian timing system.